Ever since Isaac Newton and the falling apple, surprises have often pushed physics forward. Many truths about the universe we live in and the particles that make up ourselves and the world around us, as well as the forces that drive them, seemed to come out of left field when they were first discovered. For instance, scientists once thought atoms were the smallest bits of matter in existence until they split atomic nuclei to find protons and neutrons, which in turn proved to be made of even smaller fundamental particles, called quarks. And it was less than 100 years ago that researchers found out the Milky Way wasn’t the only galaxy in the cosmos but rather one of billions.

The surprises in the history of physics are far too many to comprehensively describe, but we polled a variety of physicists for some of their favorites. A few discoveries, such as the accelerating expansion of the universe, were so groundbreaking that multiple experts picked them as top choices. And many of these events occurred relatively recently, showing that the field of physics continues to astound us. Here’s a selection of physicists’ responses on the most amazing, stunning and flabbergasting findings.

Dark Energy

One of the most shocking findings in the history of physics was the discovery of dark energy just before the turn of the millennium. None of us working in physics saw that coming! The observations that distant supernovae are dimmer than expected led to the idea that the universe is not just expanding but accelerating. These objects are very well understood, no matter how far back in time they are observed, so alternative explanations just don’t work. The name “dark energy” was given to the material that causes this acceleration. After the initial discovery, many other observations of different types confirmed this result, such as studies of the cosmic microwave background, which is the leftover light from the big bang, and studies of clusters of galaxies. The list goes on and on. We now have a standard model of cosmology in which the ordinary matter and energy that we experience in our daily lives—our body, the air we breathe, the walls around us, and all the stars and planets—add up to only 5 percent of the content of the universe. Most of the universe is “the dark side”: the universe is thought to consist of 25 percent dark matter and 70 percent dark energy. I, for one, am working to identify the nature of these mysterious components.

This discovery of dark energy in particular created a paradigm shift. The simplest explanation [for dark energy] would be a cosmological constant originally introduced by Albert Einstein as a possible term in the equations of the general theory of relativity but then abandoned by him as his “biggest blunder.” Now it seems he may have been right after all. The trouble is that the predicted value for the cosmological constant from calculations using quantum field theory produces a number that is too large by a factor of 10120. Editor’s Note: If the constant was this large, the universe would have expanded much, much faster than it did.] This conundrum has been known for some time, and theorists conjectured that there must be some physics that drives the number down to zero instead [to match the observed expansion history of the universe]. Now with the discovery of dark energy, however, the number must be driven down to a particular tiny value [rather than zero to explain the accelerating expansion], which is much harder to explain. This cosmological constant problem is thought by many to be the deepest unsolved problems in all of modern physics.” —Katherine Freese, University of Texas at Austin

Expanding Universe

I think the accelerating expansion of the universe has to be a strong contender. I’ve read references published around 1990 that talk confidently about how we will soon use supernovae to measure the rate at which the expansion of the universe is decelerating and the curvature of the cosmos and how this will tell us about the ultimate fate of our universe (because closed matter-dominated universes undergo a ‘big crunch,’ while open ones expand forever)—very little of which applies to the dark-energy-dominated, spatially-flat cosmos that we appear to actually live in! I think this also qualifies because even with the benefit of hindsight, it still seems very surprising that the dark energy/cosmological constant has its measured value. —Tracy Slatyer, Massachusetts Institute of Technology

Charmed Quarks and Accelerating Cosmos

The most spectacular discoveries in fundamental physics since I started graduate school in 1973 have been the following:
(1) The discovery in October 1974 of the J/psi particle, interpreted in terms of a new quark, the charmed quark, which gave dramatic confirmation to the then emerging Standard Model of particle physics.
(2) The discovery in the late 1990s that the expansion of the universe is accelerating, apparently because of a tiny but nonzero energy density of the vacuum, upending many of our ideas about the cosmos. —Edward Witten, Institute for Advanced Study, Princeton, N.J.

Black Holes

One of the most surprising discoveries in the history of physics is Karl Schwarzschild’s black hole solution of the Einstein equation. [Editor’s Note: Schwarzschild calculated the first exact solution to Einstein’s field equation of general relativity, and the solution predicted the existence of black holes.]

It is apocryphally said that when Einstein discovered his highly nonlinear equation, he thought an exact solution would never be found, but Schwarzschild proved him wrong only months later. Yet the structure of the solution was so surprising that many thought black holes did not exist. Einstein himself wrote in 1939 that [“the ‘Schwarzschild singularities’ do not exist in physical reality”]. It is only a century later, with the recent direct LIGO [Laser Interferometer Gravitational Wave Observatory] and EHT [Event Horizon Telescope] observations of black holes that the last shreds of disbelief have been stamped out.” —Andrew Strominger, Harvard University


It’s got to be the flexibility of spacetime. Let’s say I hop on a really fast rocket or go very close to a black hole and then return to where I started. If I go fast enough on the rocket or go close enough to the black hole, I can have only 10 minutes go by on my watch while 10,000 years go by for Earthlings. This is an experimentally verified time machine that lets you travel to the future! —Edgar Shaghoulian, University of California, Santa Cruz


I think my favorite event in physics was the prediction of the existence of the neutrino [a subatomic particle with no charge and very little mass] because so much of our fundamental approach to physics today grew out of that moment. The neutrino prediction by Wolfgang Pauli was one of the first examples of taking energy and momentum conservation seriously—you must either explain nuclear beta decay [a common radioactive process] by violating this conservation law or by introducing a new particle. The neutrino would be the first new particle predicted that wasn’t obvious in everyday life. Today predictions for new ghostlike particles are almost a dime a dozen, but in the early part of the last century, introducing potentially unobservable particles simply wasn’t done. When Enrico Fermi introduced the interaction explaining why the neutrino was so unlikely to be observed, he predicted the first new force [the weak nuclear force] beyond the two that are obvious in everyday life (gravity and electromagnetism). Today physicists consider many new types of forces all the time, but back then that just wasn’t in the picture. The idea of unifying forces, which is so essential to physics today, grew out of the discovery of Fermi’s ‘weak force’ that the neutrino feels. One of the most amazing examples that shows quantum mechanics makes sense as a theory, because it can happen on kilometer scales, where we can really see it, comes from neutrino physics. So that moment, when Pauli predicted the neutrino, is my favorite surprise because of all the paths it led to in physics. —Janet Conrad, Massachusetts Institute of Technology


I would say the discovery of neutrino oscillations is up there for me. Neutrinos themselves were predicted to exist by Pauli and subsequently discovered in a great demonstration of the power of theory. But what makes neutrinos incredibly interesting little particles is the fact that they have mass and can change flavors, which requires a modification of the Standard Model of particle physics. —Sanjana Curtis, University of Chicago


Long ago two ancient Greek savants, Democritus and Leucippus, argued that matter consists of atoms, a notion that would be confirmed more than two millennia later. I recently coined the word ‘leucippity’ to characterize those speculative hypotheses that wait many years for widespread acceptance. My new word honors the elder of the two proponents of the atomic hypothesis, Leucippus.

Isaac Newton concluded that light consists of particles in 1672; Christiaan Huygens developed his wave theory of light six years later. Who got it right? The question lingered for two centuries until James Clerk Maxwell’s profound and leucippitous discovery that light favors Huygens’s wave theory. (Later on Einstein would have his say on this matter.) Leucippity abounds in science. Alfred Wegener’s prescient ‘geopoetry’ of drifting continents emerged as the mature science of plate tectonics half a century afterward. More recently, the discovery of a boson [the Higgs boson] first imagined by Peter Higgs and a few others in 1964 was triumphantly announced at CERN [the European laboratory for particle physics near Geneva] on July 4, 2012. Lastly, the gravitational waves produced by mergers of black-hole pairs were detected by LIGO in 2015, a full century after their existence had been proposed by Einstein. Leucippity again! —Sheldon Lee Glashow, Harvard University

Phase Transitions

In my opinion, one of the most incredible and surprising experimental findings in physics resulted from when the pioneer of helium liquefaction, Heike Onnes, performed experiments in which he cooled metals such as gold, platinum and mercury to liquid helium temperatures. On the same day that he found that the electrical resistance of mercury dropped to effectively zero at liquid helium temperatures, he also found that [using a vacuum pump] on a normal liquid helium sample caused the liquid to further cool and aggressively boil before suddenly becoming placid. This is incredible! On the same day Onnes discovered both the phase transition to a state of superconductivity in mercury and the phase transition to the state of superfluidity in helium. —Charles Brown, Yale University

Bell and Michelson-Morley

Two discoveries—Bell’s theorem and the Michelson-Morley interferometry experiment—upended our understandings of space, time and the nature of reality, so I can’t resist voting for them both.

The American Physical Society calls the Michelson-Morley experiment “what might be regarded as the most famous failed experiment to date.” Until the experiment was performed in 1887, scientists believed that light waves propagate through a medium that scientists called the luminiferous aether. After all, sound waves propagate through air, and surfers’ waves propagate through water. But Albert Michelson and Edward Morley provided strong evidence that light is different; it needs no medium. This lack paved the path for Einstein’s special theory of relativity (nothing can travel more quickly than light, E = mc2 [the c stands for the speed of light in a vacuum], how short an object looks depends on how quickly you’re moving relative to it, etcetera), which led to his general theory of relativity (spacetime has a shape).

Bell’s theorem [named after John Stewart Bell] revealed that quantum systems have wonky relationships with information and with each other. Ordinarily, if you know everything about a pair of systems—say, everything about a pair of people named Audrey and Baxter—then you know everything about each individual—everything about Audrey and everything about Baxter. But if Audrey and Baxter are labels of quantum particles, then you can know everything about the pair without knowing anything about the individuals. Information can be not in one particle and not in the other but sort of in the relationship between the two: the whole is greater than the sum of its parts in quantum physics. Bell’s insight paved the path for the quantum computers and networks now under construction across the world. —Nicole Yunger Halpern, University of Maryland, author of Quantum Steampunk

Top Five

Here are a few surprising discoveries that pop into my mind, in no particular order:

(1) Special relativity: the fact that the speed of light is constant, irrespective of the frame of reference.

(2) General relativity: the fact that gravity represents a curvature of spacetime.

(3) The expansion of the universe, the ensuing big bang model and the fact that the expansion is accelerating.

(4) The ‘unreasonable’ effectiveness of mathematics in formulating the fundamental laws of nature.

(5) The probabilistic nature of quantum mechanics. —Mario Livio, astrophysicist